The present invention relates to a method of producing hard foamed materials having a small specific gravity and an increased mechanical strength by the addition of a large amount of inorganic powdery substances.
Hitherto, vinyl chloride resin foamed bodies have been widely used as a heat insulating material because of their excellent flame retarding property, chemical resistance and mechanical strength. However, there is a drawback that the foamed bodies are relatively weak to heat so that volume contraction is caused when they are exposed to a higher temperature or hydrogen chloride gas is produced when they are subjected to flame. Furthermore, they are expensive in the production cost as compared with other foamed plastics such as polyurethane and the like, so that they have been used in a relatively specific field irrespective of their excellent properties. Therefore, a method has been studied wherein polyvinyl chloride resin is mixed with a large amount of inorganic powdery substances such as calcium carbonate, calcium sulfate and the like in order to decrease the production cost and the volume contraction when they are heated, wherein the mechanical strength is further improved, and wherein the generation of hydrogen chloride gas is lowered even if the foamed body is heated or subjected to flame, and the like. For instance, a method has been developed wherein the vinyl chloride resin is mixed with an inorganic powdery substance having therein molecule bonding water and the resulting mixture is heated to generate steam and to form a foamed body.
However, the vinyl chloride resin does not show a clear melting point different from, for example, polyolefin and is liable to decompose by the heating. Further, when the vinyl chloride resin is mixed with a large amount of inorganic powdery substances, these powders can not sufficiently and uniformly be dispersed in the resulting mixture, so that it is difficult to thoroughly effect gelation for producing a foamed body. For example, when the inorganic powdery substances are used in an amount of not less than 25%, vinyl chloride resin foamed body having a specific gravity of not more than 0.3 can not be obtained.